The first time a pregnant woman feels her baby kick can be surprising -- a sudden reminder¹ that the tiny creature growing inside her has a mind of its own. But why do babies kick?

孕妇第一次感受到胎儿踢腿可能会觉得很惊奇--这是突然提醒她，在她体内生长的小生命开始有自己的想法了。但为什么胎儿会踢腿呢？

Though the womb is a tight space in which to exercise, it turns out that those kicks are vital for the baby's healthy bone and joint development, an expert told Live Science.

一名专家向《生活科学》透露，尽管子宫是一个拥挤的运动场所，但这些踢腿动作对胎儿骨骼健康和关节发育至关重要。

Fetuses² begin moving in the womb about as early as 7 weeks, when they slowly bend their necks, according to a review paper published in the journal Ultrasound in Obstetrics & Gynecology.

发表在《妇产科超声》杂志上的一篇评论文章指出，胎儿最早在7周时就能在子宫中运动，那时他们会慢慢地弯曲脖子。

As the babies grow, they gradually add more movements to their repertoire³, such as hiccupping, arm and leg movements, stretching, yawning, and thumb sucking.

随着胎儿的生长，他们的动作会逐渐增多，例如打嗝、动胳膊动腿、伸展、打哈欠和吮吸拇指。

But the mom won't feel the bigger movements -- such as kicks and punches -- until 16 to 18 weeks into her pregnancy⁴, when the baby is a bit stronger.

但孕妇只有到怀孕16周至18周时才能感受到幅度较大的动作，例如踢腿和击打，此时胎儿已经更强壮了。

An entire field of research is dedicated⁵ to figuring out whether the baby is in control of its movement or if those movements are just a reflex, said Niamh Nowlan, a bioengineer at Imperial College London.

伦敦帝国理工学院生物工程学家尼娅姆·诺兰称，一个完整的研究正在致力于弄清楚是胎儿能控制这些动作，还是说这些动作只是一种反射。

"Early movements are likely to be purely⁶ reflex," Nowlan told Live Science in an email, "but as the movements become more coordinated⁷, it's likely the brain is in control of how much and when the baby moves."

诺兰在发给《生活科学》的一封电子邮件中写道：“早期的动作很可能只是纯粹的反射，但随着动作变得更加协调，很可能是大脑在控制胎儿的运动速度和时间。”

Scientists may not know for sure if the movements are voluntary or involuntary, but Nowlan said the research is clear that movement is important.

科学家们或许无法确定胎儿是主动还是无意识做出这些动作的，但诺兰说，研究明确了一点：运动很重要。

"The baby needs to move in the womb to be healthy after birth, particularly for their bones and joints⁸," she said.

她说：“胎儿需要在子宫里运动以保持健康，尤其是对他们的骨骼和关节而言。”

In a review she published in the journal European Cells and Materials, Nowlan described how a lack of fetal movement can lead to a variety of congenital disorders⁹, such as shortened joints and thin bones that are susceptible¹⁰ to fracture.

诺兰在期刊《欧洲细胞和材料》上发表的一篇评论性文章描述了缺乏胎动是如何导致各种先天疾病的，例如关节短和骨骼细，这些问题容易引发骨折。

For pregnant women wondering if their baby is too kicky, or not kicky enough, Nowlan said there's no established amount of normal fetal movement during pregnancy. "Pregnant women are told to look out for significant changes in movements, which is quite vague advice, but it's the best that can be given at the moment," she said.

对于那些担心宝宝胎动太多或是太少的孕妇，诺兰表示，在孕期胎儿运动并没有固定的量。她说：“孕妇被告知要注意胎动的明显变化，这是一个非常模糊的建议，但也是当前能提供的最好的建议。”

That's because it's difficult for scientists to study fetal movements, because the only way to measure them is in the hospital and it can be done for only a short period at a time. It's unlikely that each woman will feel the same thing when her baby starts kicking.

这是因为科学家很难研究胎儿的运动，要监测它们的唯一方式就是在医院里，而且只能在短时间内完成。当胎儿开始踢腿时，每个准妈妈的感觉可能不同。

"Different women experience the sensation quite differently, and sensations can vary between pregnancies," Nowlan said.

诺兰说道：“不同的女性对这种感觉的感受是完全不同的，而且同一个人每次怀孕期间的感觉也会有所不同。”

Feeling a baby moving and kicking in the womb might be a weird¹¹ sensation, but it's simply a sign of healthy development.

感受胎儿在子宫中动动和踢腿或许是种怪怪的体验，但这其实不过是健康发育的迹象。